Optical pickup unit and information recording and reproduction apparatus

ABSTRACT

An optical pickup unit includes a light source emitting a light beam, an objective lens focusing the light beam onto an information recording medium, a light detection part receiving the light beam reflected from the information recording medium, and a light blocking part selectively blocking a part of the light beam with respect to a radial direction. The light blocking part is provided in an optical path of the light beam centered on an optical axis.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to optical pickup unitsand information recording and reproduction apparatuses, and moreparticularly to an optical pickup unit recording information on andreproducing information from information recording media and aninformation recording and reproduction apparatus including such anoptical pickup unit.

[0003] 2. Description of the Related Art

[0004] Disk-type optical information recording media widely usednowadays are compact disks (CDs) such as CD-ROMs, CD-Rs, and CD-RWs anddigital versatile disks (DVDs) such as DVD-ROMs, DVD-Rs, and DVD-RWs.Some CDs have realized a recording density of 650 MB. The DVDs arelarger in capacity than the CDs, but have yet to satisfy the demands ofusers in terms of capacity. In this context, a so-called multilayerdisk, a disk formed of a plurality of recording layers instead of asingle recording layer, has been developed for achieving higherrecording density.

[0005] In such an information recording medium of multiple recordinglayers, the recording layers are required to be separated from eachother by tens of micrometers (μm) or more so that information may berecorded on and reproduced from each recording layer independently.However, distances from an objective lens to the recording layers aredifferent so that spherical aberration occurs in recording layers out ofan optimum position. That is, in such recording layers, the position ofthe focus of marginal rays (rays entering the periphery of a lens awayfrom the optical axis thereof) entering a lens 90 shown in FIG. 1 isdeviated in the direction of the optical axis from the position of thefocus of paraxial rays (rays entering the central part of a lens)entering the lens 90.

[0006]FIG. 2 is a diagram for illustrating focusing of a light beam inthe case of reproducing information from a multilayer disk by a singleconventional optical pickup unit. In the case of reproducing informationfrom a first recording layer 101 a of a multilayer disk 101, the lightbeam is focused on the first recording layer 101 a through an objectivelens of the optical pickup unit at a position indicated by 100 a asshown on the left side in FIG. 2. In the case of reproducing informationfrom an n^(th) recording layer 101 n, which is farther away from a disksubstrate surface 101 s than the first recording layer 101 a is, thelight beam is focused on the n^(th) recording layer 101 n through theobjective lens at a position indicated by 100 b as shown on the right inFIG. 2.

[0007] If the optical pickup unit is optimized for focusing the lightbeam into a spot on the first recording layer 101 a to reproduce theinformation therefrom, the optical pickup unit has no problem inreproducing the information from the first recording layer 101 a.However, when the objective lens of the optical pickup unit gets closerto the disk substrate surface 101 s of the multilayer disk 101 toreproduce the information from the n^(th) recording layer 101 n, and thelight beam is focused into a spot on the n^(th) recording layer 101 n,spherical aberration occurs due to an interlayer thickness between thefirst and n^(th) recording layers 101 a and 101 n. As a result, the spotformed on the n^(th) recording layer 101 n is larger in diameter thanthe spot formed on the first recording layer 101 a as shown in FIG. 2.

[0008] In order to solve this problem, it is necessary to develop a newoptical head control technology of performing aberration correction bymeasuring the amount of spherical aberration of a beam spot. JapaneseLaid-Open Patent Application No. 2000-155979 discloses an aberrationdetection device as means for solving this problem.

[0009]FIG. 3 is a diagram showing a configuration of the aberrationdetection device. As shown in FIG. 3, a light beam emitted from a lightsource 201 and reflected from an optical disk 206 is split by a halfmirror 202 to be divided into a light beam passing through a specificregion and a light beam passing through the other regions by a hologram209. The light beam passing through the specific region is deflected bythe hologram 209 to be received by a plurality of photodetectors 207 sothat the photodetectors 207 obtain respective signals. The obtainedsignals are compared so that an aberration is detected. The detectedaberration is transmitted via an aberration signal processing circuit208 to an aberration correction device 204 so that the aberrationcorrection device 204 can be driven in real time based on the aberrationso as to reduce the aberration of the optical system. In FIG. 3,reference numerals 203 and 205 denote a collimator lens and an objectivelens, respectively.

[0010] The assembly conditions of a light-receiving element in theoptical pickup unit are extremely strict so that the light-receivingelement is required to be provided with an accuracy of a few micrometersor less. This reduces yield rate, thereby affecting the cost to aconsiderable extent. Further, the aging and temperature characteristicsof the light-receiving element are subject to change. Therefore, it isdesirable that a light-receiving element pattern be simple. However,according to the technology disclosed in Japanese Laid-Open PatentApplication No. 2000-155979, an incident light beam for aberrationdetection is split into a plurality of beams by a hologram so that aplurality of light-receiving elements detect a given one of the splitbeams. Such a configuration, where each split beam is focused into asmall spot and a plurality of light-receiving elements detect a givenone of the split beams, is complicated and may impair the stability ofthe optical pickup unit. This may reduce yield rate, thereby incurringan increase in cost.

[0011] The optical pickup unit is known as a device for recordinginformation on and reproducing recorded information from an informationrecording medium. The ratio of the intensity of a light beam focusedonto the information recording medium for recording to that forreproduction ranges from 5:1 to 15:1. Generally, the emission power of alight source switches between recording time and reproduction timesubstantially in accordance with the ratio. However, in somesemiconductor lasers, noise characteristics worsen as an output lowers.

[0012] That is, part of a light beam emitted from a semiconductor laserreturns to the semiconductor laser as a returning light. A differentresonator other than the semiconductor laser is formed between thereturning light and the information recording medium. As a result, thestate of oscillation of the semiconductor laser becomes unstable so thatthe output of the semiconductor laser includes noise. Further, noise isalso generated in the output of the semiconductor laser by the operationof the semiconductor laser or a variation in environmental temperature.When the light-emission power of the semiconductor laser is far above athreshold current level to reach tens of milliwatts (mW), the lightemission of the semiconductor laser is stable, being hardly affected bydisturbances. However, when the light-emission power of thesemiconductor laser is around the threshold current level, thelight-emission power is affected by disturbances including those causedby the above-described returning light, so that variations are caused inthe light-emission power.

[0013] Accordingly, in the case of recording information on (writinginformation to) the information recording medium or erasing informationrecorded thereon, the light emission of the semiconductor laser ishardly affected by disturbances since the light-emission power of thesemiconductor laser is far above the threshold current level. However,in the case of reproducing (reading out) information from theinformation recording medium, the light-emission power of thesemiconductor laser is normally set to a low level so that thesemiconductor laser emits the laser beam with a power of a fewmilliwatts slightly over the threshold current level, for instance, apower of five milliwatts. In this case, therefore, the semiconductorlaser is especially subject to the returning light to be unstable inemitting the laser beam. Accordingly, the signal is deteriorated bynoise caused in the output of the semiconductor laser.

[0014] Japanese Laid-Open Patent Application No. 9-27141 discloses anoptical pickup unit to solve this problem. According to this opticalpickup unit, an electro-optical element capable of controllingtransmittance of light is provided in an optical path from a lightsource to a recording medium. The electro-optical element controlstransmittance for light emitted from the light source to a low rate(value) at a time of reproducing information, and to a high rate (value)at a time of recording information.

[0015] Further, in an optical pickup unit, light emitted from asemiconductor laser is focused onto a surface of an informationrecording medium through a focus optical system, and a reflected lightfrom the information recording medium is directed through a detectionoptical system to a light-receiving element. Generally, the detectionsignal (electric current signal) of the light-receiving element of theoptical pickup unit is converted into a voltage signal by acurrent-voltage conversion amplifier housed in the optical pickup unitto be output to-a signal processing circuit.

[0016] If the amplitude level of the signal output through thecurrent-voltage conversion amplifier is too low, a problem is caused ininformation reproduction. Therefore, such a configuration is employedthat the gain of the current-voltage conversion amplifier is switchableso that the output amplitude level thereof falls within a proper range.However, in recent years, it has been required for the optical pickupunit to accommodate a plurality of conditions so as to be suitable for avariety of types of information recording media and various recordingand reproduction conditions. Accordingly, in order to performinformation recording, reproduction, and erasure in compliance with aplurality of types of optical disks, such as a CD-RW (compact diskrewritable), a CD-DA (compact disk digital audio), a CD-ROM, and a CD-R(compact disk recordable), as performed by an optical informationrecording and reproduction apparatus disclosed in Japanese Laid-OpenPatent Application No. 10-255301, it is necessary to make such a gainswitching circuit shown in FIG. 4 suitable for more combination patternsof disk types and light powers. Consequently, the number of resistorsemployed in the gain switching circuit increases so that the speed ofresponse of a signal is reduced.

[0017] In addition, recently, it has been proposed to increase thenumerical aperture (NA) of an objective lens for focusing a light beamonto an information recording medium so as to achieve high recordingdensity. This is because the diameter of a beam spot can be reduced byusing an objective lens of a large NA. However, as the NA increases, therate of increase of aberration also increases. That is, sphericalaberration, whose primary cause is a substrate thickness error in theinformation recording medium, is proportional to the NA to the fourthpower, and coma, whose primary cause is the inclination of theinformation recording medium to the optical axis, is proportional to thecube of the NA.

[0018] Japanese Laid-Open Patent Application Nos. 10-20263, 9-128785,11-259892, and 2000-155979 disclose techniques to correct and detectwavefront aberration (spherical aberration, coma, and astigmatism),which techniques are known as prior art for solving the above-describedproblems.

[0019] However, optical pickup units having such configurations asdisclosed in the above-described references need to have their costsreduced by component sharing and their assembly processes simplified.

SUMMARY OF THE INVENTION

[0020] Accordingly, it is a general object of the present invention toprovide an optical pickup unit and an information recording andreproduction apparatus in which the above-described disadvantages areeliminated.

[0021] A more specific object of the present invention is to provide anoptical,pickup unit that can stably perform spherical aberrationdetection, being made less subject to the effects of variations in itsaging and temperature characteristics by providing a light-blocking partusing a liquid crystal element in a substantially parallel optical pathwithout increasing the number of light-receiving elements as in theconventional optical pickup unit, the optical pickup unit also realizingcost reduction by reducing the number of components and the number ofassembly processes.

[0022] Another more specific object of the present invention is toprovide an optical pickup unit including an electro-optical elementdividing the region of a light beam passing therethrough so as to becapable of performing stable control without its light source beingaffected by noise at a time of reproduction, processing a lightreception signal without decreasing response speed, and performingcorrections based on wavefront aberration, the optical pickup unitrealizing component sharing, reduction in the number of components, andsimplification of assembly adjustment.

[0023] Yet another more specific object of the present invention is toprovide an information recording and reproduction apparatus includingany of the above-described optical pickup units.

[0024] The above-objects of the present invention are achieved by anoptical pickup unit including a light source emitting a light beam, anobjective lens focusing the light beam onto an information recordingmedium, a light detection part receiving the light beam reflected fromthe information recording medium, and a light blocking part selectivelyblocking a part of the light beam with respect to a radial direction,the light blocking part provided in an optical path of the light beam tobe centered on an optical axis.

[0025] The above-described optical pickup unit is capable of detectingrays around the axis of a beam spot and the rays of the peripheral partof the beam spot separately. Therefore, spherical aberration, which is aphenomenon of a difference between focus positions, can be detectedbased on the detection signals of the rays.

[0026] Additionally, the above-described optical pickup unit may furthercomprise a control part generating an aberration signal by comparing afirst signal generated based on a first part of the light beam passingthrough a first region of the light blocking part and a second signalgenerated based on a second part of the light beam passing through asecond region of the light blocking part, the first region of the lightblocking part being provided internal to the second region thereof.

[0027] Thereby, an amount of correction of spherical aberration can beset by using the aberration signal.

[0028] Additionally, the above-described optical pickup unit may furthercomprise a spherical aberration correction part correcting sphericalaberration based on the aberration signal generated by the control part.

[0029] Thereby, the spherical aberration can be corrected based on theamount of correction obtained from the aberration signal.

[0030] Additionally, in the above-described optical pickup unit, thelight blocking part and the spherical aberration correction part mayform a single element.

[0031] Thereby, the above-described optical pickup unit, which can berealized with reduced space, has its number of components reduced sothat cost reduction thereof can be realized.

[0032] The above objects of the present invention are also achieved byan information recording and reproduction apparatus including an opticalpickup unit that includes a light source emitting a light beam, anobjective lens focusing the light beam onto an information recordingmedium, a light detection part receiving the light beam reflected fromthe information recording medium, and a light blocking part selectivelyblocking a part of the light beam with respect to a radial direction,the light blocking part provided in an optical path of the light beam tobe centered on an optical axis.

[0033] According to the above-described optical pickup unit, rays aroundthe axis of a beam spot and the rays of the peripheral part of the beamspot can be detected separately. Therefore, spherical aberration, whichis a phenomenon of a difference between focus positions, can be detectedbased on the detection signals of the rays.

[0034] The above objects of the present invention are also achieved byan optical pickup unit performing recording or reproducing informationby making a light beam emitted from a light source incident on aninformation recording medium, the optical pickup unit including anelectro-optical element switching values of transmittance of a givenregion through which the light beam passes depending on whether theinformation is recorded or reproduced.

[0035] Additionally, in the above-described optical pickup unit, theelectro-optical element may be provided subsequent to the light sourcein an optical path between the light source and the informationrecording medium, and the transmittance of the given region of theelectro-optical element may be controlled to a first value at a time ofreproducing the information from the information recording medium and toa second value at a time of recording the information on the informationrecording medium, the first value being smaller than the second value.

[0036] Additionally, in the above-described optical pickup unit, theelectro-optical element may include an outer region provided outside thegiven region so that the light beam passing through the outer region hasa phase difference thereof selectively varied in the outer region.

[0037] According to the above-described optical pickup unit, stablecontrol can be performed without noise affecting the light beam passingthrough the given region. Further, the light beam passing through theouter region has its phase difference selectively varied so thatdeterioration of the wave surface of the light beam incident on theinformation recording medium can be suppressed. Thereby, the spotperformance of the light beam collected by an objective lens can besecured.

[0038] Additionally, in the above-described optical pickup unit, thelight beam passing through the outer region may have the phasedifference thereof selectively varied concentrically in the outerregion.

[0039] Additionally, in the above-described optical pickup unit, thelight beam passing through the outer region may have the phasedifference thereof varied in the outer region step by step in a radialor a tangential direction.

[0040] Additionally, in the above-described optical pickup unit, thelight beam passing through the outer region may have the phasedifference thereof varied in the outer region simultaneously andasymmetrically in radial and tangential directions.

[0041] Thereby, wavefront aberration (spherical aberration, coma, andastigmatism) can be suppressed.

[0042] The above objects of the present invention are further achievedby an information recording and reproduction apparatus recordinginformation on and reproducing information from an information recordingmedium, the information recording and reproduction apparatus includingan optical pickup unit performing recording or reproducing informationby making a light beam emitted from a light source incident on theinformation recording medium, the optical pickup unit including anelectro-optical element switching values of transmittance of a givenregion through which the light beam passes depending on whether theinformation is recorded or reproduced.

[0043] According to the above-described information recording andreproduction apparatus, information recording and reproduction can bestably performed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings, in which:

[0045]FIG. 1 is a diagram for illustrating spherical aberration;

[0046]FIG. 2 is a diagram for illustrating focusing of a light beam in acase of reproducing information from a multilayer disk by a singleconventional optical pickup unit;

[0047]FIG. 3 is a schematic diagram showing an optical system of anoptical pickup unit including a conventional aberration detectiondevice;

[0048]FIG. 4 is a diagram showing a conventional gain switching circuit;

[0049]FIG. 5 is a schematic diagram showing an optical system of anoptical pickup unit according to a first embodiment of the presentinvention;

[0050]FIG. 6 is a plan view of a turning part of the optical pickup unitaccording to the first embodiment, showing transparent electrodepatterns formed thereon;

[0051]FIG. 7 is a sectional view of the turning part of FIG. 6;

[0052]FIG. 8 is a plan view of a spherical aberration correction part ofthe optical pickup unit according to the first embodiment, showingtransparent electrode patterns formed thereon;

[0053]FIGS. 9A through 9G are diagrams for illustrating correction ofspherical aberration in an information recording and reproductionapparatus including the optical pickup unit according to the firstembodiment;

[0054]FIG. 10 is a diagram for illustrating a configuration of theoptical pickup unit of the first embodiment in which another sphericalaberration correction element is employed;

[0055]FIG. 11 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a secondembodiment of the present invention;

[0056]FIG. 12 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a third embodimentof the present invention;

[0057]FIG. 13 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a fourthembodiment of the present invention;

[0058]FIG. 14 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a fifth embodimentof the present invention;

[0059]FIG. 15 is a perspective view of an information recording andreproduction apparatus according to a sixth embodiment of the presentinvention;

[0060]FIG. 16 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a seventhembodiment of the present invention;

[0061]FIG. 17 is a sectional view of a first electro-optical element(liquid crystal cell) of the optical pickup unit of FIG. 16;

[0062]FIG. 18 is a diagram showing a relationship between transmittanceand applied voltage in the liquid crystal cell of FIG. 17;

[0063]FIG. 19 is a diagram showing a transmittance change region (afirst region) and an aberration correction region (a second region) ofthe first electro-optical element of FIG. 17;

[0064]FIGS. 20A through 20C are diagrams showing spherical aberration,coma, and astigmatism, respectively;

[0065]FIG. 21 is another diagram showing the spherical aberration;

[0066]FIG. 22A is a diagram showing a phase of a light beam at a time ofoccurrence of the spherical aberration, and FIG. 22B is a diagramshowing a pattern of an electro-optical element for selectivelytransmitting the light beam;

[0067]FIG. 23A is a diagram showing a section of the sphericalaberration and a phase difference for correcting the sphericalaberration, and FIG. 23B is a diagram showing the spherical aberrationafter correction;

[0068]FIGS. 24A through 24C are diagrams showing patterns for correctingthe spherical aberration, coma, and astigmatism, respectively, in theelectro-optical element;

[0069]FIG. 25A is a diagram showing a phase of the light beam at a timeof occurrence of the coma, and FIG. 25B is a diagram showing a patternof the electro-optical element for selectively transmitting the lightbeam; and

[0070]FIG. 26A is a diagram showing a section of the coma and a phasedifference for correcting the coma, and FIG. 26B is a diagram showingthe coma after correction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0071] A description will now be given, with reference to theaccompanying drawings, of embodiments of the present invention.

[0072]FIG. 5 is a schematic diagram showing an optical system of anoptical pickup unit according to a first embodiment of the presentinvention. The optical system of the optical pickup unit of FIG. 5includes a multilayer information recording medium 1, a semiconductorlaser 2 emitting a p-polarized laser beam, a collimator lens 3collimates the laser beam emitted from the semiconductor laser 2 intoparallel rays, a beam splitter 4 letting through the laser beam from thecollimator lens 3 and deflecting a reflected light from the multilayerinformation recording medium 1, a deflection mirror 5 deflecting thelaser beam, an objective lens 6 gathering the laser beam incidentthereon with its optical characteristics such as numerical aperture andspherical aberration being optimized for one of the layers of themultilayer information recording medium 1, a turning part 7 thatselectively turns the plane of polarization of the reflected light fromthe multilayer information recording medium 1 deflected by the beamsplitter 4, a polarizing plate 8 transmitting the p-polarized reflectedlight, a detection lens 9 gathering the reflected light from thepolarizing plate 8, a light-receiving element 10 receiving the reflectedlight to output a tracking signal, a focus signal, and a reproductionsignal, a spherical aberration detection part 11 detecting sphericalaberration from the output of the light-receiving element 10, aspherical aberration correction part 12, and a spherical aberrationcontrol circuit 13 driving and controlling the spherical aberrationcorrection part 12 based on the output of the spherical aberrationdetection part 11.

[0073] As shown in FIG. 6, the turning part 7 is formed of a liquidcrystal element composed of a central region A and a peripheral regionB. Voltage is applied to a selected one of the central region A and theperipheral region B so that the direction of polarization of the lightbeam passing through the central region A or the peripheral region B canbe changed by the applied voltage.

[0074] As shown in FIG. 7, the turning part 7 has a twisted nematic (TN)liquid crystal 7 a sandwiched, in the direction of the optical axis ofthe turning part 7, between two glass plates 7 b and 7 c withtransparent electrodes. When a control voltage (control signal) of zerovolts is applied (that is, no voltage is applied) between an electrodeof the upper glass plate 7 b (hereinafter referred to simply as an upperelectrode) and an electrode of the lower glass plate 7 c (hereinafterreferred to simply as a lower electrode), the plane of polarization ofthe laser beam is turned 90° by the turning part 7. On the other hand,when a voltage higher than a threshold voltage, such as a controlvoltage (control signal) of five volts, is applied between the upper andlower electrodes, the laser beam passes through the turning part 7 as itis without its plane of polarization being turned.

[0075] Next, a description will be given of an operation of the turningpart 7.

[0076] First, a voltage higher than the threshold voltage is appliedbetween the upper and lower electrodes of the central region A of theturning part 7, while a voltage of zero volts is applied to theperipheral region B. Thereby, the light beam passing through the turningpart 7 has its plane of polarization remaining unchanged in its partpassing through the central region A and turned 90° in its part passingthrough the peripheral region B. Under this setting condition, thep-polarized laser beam is emitted from the semiconductor laser 2 andreflected from the multilayer information recording medium 1 to beincident on the turning part 7 through the beam splitter 4.

[0077] Since the voltage higher than the threshold voltage is applied tothe central region A, the light beam passes through the central region Ato remain p-polarized with its plane of polarization remainingunchanged. On the other hand, since the voltage of zero volts is appliedto the peripheral region B, the light beam has its plane of polarizationturned by 90° in the peripheral region B so that an s-polarized light isemitted therefrom. Thereafter, the laser beam is incident on thepolarizing plate 8. Since the polarizing plate 8 lets through ap-polarized light, the laser beam from the central region A of theturning part 7 passes through the polarizing plate 8, while thes-polarized light beam from the peripheral region B of the turning part7 is prevented from passing through the polarizing plate 8.

[0078] That is, the laser beam from the central region A of the turningpart 7 passes through the polarizing plate 8 as it is to be incident onthe detection lens 9, but the laser beam from the peripheral region B ofthe turning part 7 is blocked off by the polarizing plate 8 and is notincident on the detection lens 9. The laser beam from the central regionA of the turning part 7 is collected by the detection lens 9 to bereceived by the light-receiving element 10 so that a focus position isoutput from the light-receiving element 10.

[0079] Next, the voltage applied to the central region A and the voltageapplied to the peripheral region B are switched so that the voltage ofzero volts is applied between the upper and lower electrodes of thecentral region A and the voltage higher than the threshold voltage isapplied to the peripheral region B. In this case, contrary to theprevious case where the voltage higher than the threshold voltage isapplied to the central region A and the voltage of zero volts is appliedto the peripheral region B, the laser beam from the peripheral region Bof the turning part 7 is incident on the light-receiving element 10. Afocus position is output from the light-receiving element 10 based onthe incident light beam.

[0080] The signals of focus position (focus position signals) outputfrom the light-receiving element 10 are input to the sphericalaberration detection part 11. The spherical aberration detection part 11obtains a difference between the focus position signals output from thelight-receiving element before and after the switching of the voltagesapplied to the turning part 7. The spherical aberration detection part11 outputs the difference to the spherical aberration control circuit13.

[0081] Here, spherical aberration is a phenomenon that the focusposition of rays around the axis of a beam spot (paraxial rays) isdifferent from the focus position of rays of the peripheral part of thebeam spot (marginal rays). Therefore, an amount of spherical aberrationcan be obtained from a difference between the focus positions bydetecting the focus positions individually by separating the paraxialrays from the marginal rays. According to the first embodiment, thespherical aberration detection part 11 is capable of detecting the focuspositions of the two light beams separately. The spherical aberrationcontrol circuit 13 generates a control signal proportional to the amountof spherical aberration based on the difference between the two focuspositions. The spherical aberration control circuit 13 drives andcontrols the spherical aberration correction part 12 based on thecontrol signal so that the spherical aberration is corrected.

[0082] Next, a more detailed description will be given of the sphericalaberration correction part 12.

[0083] The spherical aberration correction part 12 includes two glasssubstrates with transparent electrodes and liquid crystal moleculessandwiched between the glass substrates. An upper one of the two glasssubstrates (hereinafter, an upper glass substrate) has concentricelectrode patterns formed thereon of transparent electrodes as shown inFIG. 8. Electrodes are formed on a lower one of the two glass substrates(hereinafter, a lower glass substrate) so as to oppose the concentricelectrode patterns formed on the upper glass substrate. The concentrictransparent electrode patterns may be formed on the lower glasssubstrate instead of the upper glass substrate.

[0084] When driving voltages are applied to the transparent electrodes,the liquid crystal molecules are aligned in accordance with electricfields generated by the applied voltages. Thereby, a refractive indexdistribution can be set as desired in a section of the light beampassing through the spherical aberration correction part 12 whichsection is perpendicular to a direction in which the light beam travels.Accordingly, the wave surface of the light beam can be divided intoregions so that the phase of the wave surface can be controlledindependently in each divided region. That is, the spherical aberrationcorrection part 12 is employable for changing a refractive index.

[0085] Therefore, by variably controlling the voltage applied to each ofthe transparent electrode patterns formed on the upper or lower glasssubstrate, spherical aberration caused by a distance or an interlayerthickness between the recording layer for which the optical pickup unitis optimized and a recording layer from which information is to bereproduced can be corrected. Thereby, an optical pickup unitautomatically correcting spherical aberration can be realized.

[0086] Next, a description will be given, with reference to FIGS. 9Athrough 9G, of correction of spherical aberration in an informationrecording and reproduction apparatus including the optical pickup unitof the first embodiment. In each of FIGS. 9A through 9C, 9E, and 9F, apoint O on a horizontal axis corresponds to an optical axis, and D-D′represents positions on a straight line that perpendicularly crosses theoptical axis at the point O. For instance, D-D′ represents the pupilsurface of the objective lens. Further, a vertical axis L represents anamount of spherical aberration. FIG. 9D is a diagram showing a liquidcrystal panel on which three concentrically divided electrodes 14through 16 are formed. FIG. 9G is a diagram showing electrode patternsin the case of forming five divided electrodes on the liquid crystalpanel.

[0087]FIG. 9A is a diagram showing a distribution pattern of sphericalaberration caused by the interlayer thickness between the recordinglayer for which the optical pickup unit is optimized and the recordinglayer from which information is to be reproduced. The sphericalaberration shown in FIG. 9A is obtained by converting sphericalaberration occurring on the recording layer into spherical aberration onthe pupil surface of the objective lens by ray tracing.

[0088] Normally, the laser beam is focused into a circular beam spot sothat spherical aberration varies in a radial direction. If theinterlayer thickness increases, the two peaks of the sphericalaberration shown in FIG. 9A become higher. In the case of occurrence ofsuch spherical aberration, the liquid crystal panel of FIG. 9D iscontrolled so that voltage is applied to the electrode 15 so as toprovide, as shown in FIG. 9B, a phase difference canceling the sphericalaberration of FIG. 9A to a light beam passing through the part of theelectrode 15, while voltages are applied to the electrodes 14 and 16 sothat light beams pass through the parts of the electrodes 14 and 16 asthey are without any phase differences being provided thereto.

[0089] Thus, by applying the different voltages to the concentricallydivided electrodes 14 through 16 formed on the liquid crystal panel,correction as shown in FIG. 9B is performed on the spherical aberrationof FIG. 9A so that residual spherical aberration can be minimized asshown in FIG. 9C. Thereby, the diameter of the beam spot can be reduced.FIGS. 9E and 9F are diagrams showing a correction to the sphericalaberration and the result of the correction, respectively, in the casewhere the five concentrically divided electrodes are formed on theliquid crystal panel as shown in FIG. 9G. By controlling the liquidcrystal display of FIG. 9G so that different voltages are applied to thefive divided electrodes so as to provide phases canceling the sphericalaberration to light beams passing through given ones of the fiveelectrodes as shown in FIG. 9E, a more precise correction can be made tothe spherical aberration as shown in FIG. 9F. The residual sphericalaberration is further reduced in FIG. 9F compared with FIG. 9C, so thatthe diameter of the beam spot can be further decreased.

[0090] According to the optical pickup unit thus configured according tothe first embodiment, which unit employs the spherical aberrationdetection part 11 and the spherical aberration correction part 12, anamount of spherical aberration can be measured by the sphericalaberration detection part 11 so that the peripheral region of the laserbeam can be optimized by the spherical aberration correction part 12even if the peripheral region of the laser beam is not focused on arecording surface of the multilayer information recording medium 1 bythe effect of the spherical aberration in the case of reproducinginformation from a recording layer other than the recording layer forwhich the objective lens 6 of the optical pickup unit is optimized.Therefore, recording can be performed on the multilayer informationrecording medium 1 with high accuracy and a high-quality signal can bereproduced from the multilayer information recording medium 1. The sameeffects can be produced by the information recording and reproductionapparatus including the above-described optical unit according to thefirst embodiment.

[0091] In the optical pickup unit of FIG. 5, the turning part 7 and thepolarizing plate 8 may be formed as a single unit. This reduces thenumber of assembly steps, thus realizing cost reduction. Further, inthis case, the optical pickup unit can be further downsized bydepositing a polarizing film on the turning part 7 instead of attachingthe polarizing plate 8 thereto.

[0092] Further, the polarizing plate 8 of FIG. 5 may be replaced byanother optical device for polarization selection, such as apolarization beam splitter or a polarization hologram.

[0093] Furthermore, the spherical aberration correction part 12 of theoptical pickup unit of FIG. 5 is not limited to a liquid crystalelement, which is employed in the first embodiment.

[0094]FIG. 10 is a diagram for illustrating a configuration of theoptical pickup unit of the first embodiment in which a movable lens isemployed to perform spherical aberration correction.

[0095] The optical pickup unit of FIG. 10 includes an objective lens 20and a spherical aberration correction part 21. In FIG. 10, the sameelements as those of FIG. 5 are referred to by the same numerals, and adescription thereof will be omitted.

[0096] In the configuration of FIG. 10, the objective lens 20 composedof two aspherical lenses 20 a and 20 b replaces the objective lens 6 ofFIG. 5, and the spherical aberration correction part 21 having adistance adjustment mechanism varying a distance between the asphericallenses 20 a and 20 b replaces the spherical aberration correction part12 of FIG. 5. The distance adjustment mechanism of the sphericalaberration correction part 21 is provided between the aspherical lenses20 a and 20 b.

[0097] A piezoelectric element is applicable as the distance adjustmentmechanism. The distance between the aspherical lenses 20 a and 20 bbecomes greater by applying a higher voltage to the piezoelectricelement and smaller by applying a lower voltage thereto. Sphericalaberration can be corrected by generating such a variation in voltagebased on the output of the spherical aberration detection part 11.

[0098] According to the configuration of FIG. 10, spherical aberrationon an information recording layer can be reduced so that the opticalpickup unit has good recording and reproduction characteristics.

[0099] In the optical pickup unit of FIG. 10, an electromagneticallydriven actuator or motor may be used as the distance adjustmentmechanism instead of the piezoelectric element. An actuator driven byultrasonic waves is also employable instead of the piezoelectricelement. Further, a group of two convex lenses or a combination of anaspherical lens and a spherical lens may be employed as the objectivelens 20 instead of the two aspherical lenses 20 a and 20 b.

[0100]FIG. 11 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a secondembodiment of the present invention. The optical pickup unit of FIG. 11includes a polarization beam splitter 30 and a turning part 31. In FIG.11, the same elements as those of FIGS. 5 and 10 are referred to by thesame numerals, and a description thereof will be omitted.

[0101] The optical pickup unit of the second embodiment is differentfrom that of the first embodiment in the following points:

[0102] (a) The polarization beam splitter 30 replaces the beam splitter4 of the first embodiment.

[0103] (b) The turning part 31, which replaces the turning part 7 of thefirst embodiment, is provided in the optical path between thepolarization beam splitter 30 and the objective lens 20.

[0104] (c) The turning part 31 includes a functional elementcorresponding to a ¼ wave plate, while the turning part 7 of the firstembodiment is employed as a functional element corresponding to a{fraction (1/2 )} wave plate (that is, the turning part 7 turns a planeof polarization 90° when the voltage is applied to the turning part 7,and does not turn the plane of polarization when no voltage is appliedthereto).

[0105] (d) The polarizing plate 8 of the first embodiment is notemployed in the second embodiment.

[0106] The ¼ wave plate circularly polarizes the linearly polarizedlaser beam emitted from the semiconductor laser 2 so that the circularlypolarized light beam is incident on the multilayer information recordingmedium 1. On the other hand, the ¼ wave plate linearly polarizes areflected light beam from the multilayer information recording medium 1,which light beam is circularly polarized in the reverse direction, intoa light beam whose plane of polarization is perpendicular to that of thelight beam emitted from the semiconductor laser 2 toward the multilayerinformation recording medium 1. Unlike the non-polarization beamsplitter 4 of the first embodiment, the polarization beam splitter 30 isa polarizing element of ‘1’/‘0’ that transmits a p-polarized laser beamand reflects an s-polarized laser beam. The second embodiment employsthe same components as the first embodiment except those mentionedabove.

[0107] Like the configuration shown in FIG. 6, the turning part 31 isformed of a liquid crystal element composed of the central region A andthe peripheral region B. Voltage is applied to a selected one of thecentral region A and the peripheral region B so that the state ofpolarization of the light beam passing through the central region A orthe peripheral region B may be changed by the applied voltage.

[0108] Like the configuration of FIG. 7, the turning part 31 has aliquid crystal sandwiched, in the direction of an optical axis, betweentwo upper and lower glass plates with transparent electrodes. When avoltage lower than a threshold voltage is applied between the upper andlower electrodes in the region A or B, the ¼ wave plate function of theturning part 31 is started so that the laser beam is emitted therefromcircularly polarized. On the other hand, when a voltage higher than thethreshold voltage is applied between the upper and lower electrodes, theturning part 31 loses its refractive index anisotropy, causing no phasedifference in the laser beam with Δn=0. Therefore, the laser beam passesthrough the turning part 31 as it is without its plane of polarizationbeing turned.

[0109] In the case of recording information on or reproducinginformation from the multilayer information recording medium 1, first,the voltage higher than the threshold voltage is applied between theupper and lower electrodes of the central region A, while the voltagelower than the threshold value is applied to the peripheral region B.Thereby, the light beam passing through the turning part 31 has itsplane of polarization remaining unchanged in its part passing throughthe central region A and circularly polarized in its part passingthrough the peripheral region B. Under this setting condition, thep-polarized laser beam is emitted from the semiconductor laser 2 to beincident on the turning part 31.

[0110] Since the voltage higher than the threshold voltage is applied tothe central region A, the incident laser beam passes through the centralregion A as the p-polarized light with its plane of polarizationremaining unchanged. On the other hand, since the voltage lower than thethreshold voltage is applied to the peripheral region B, the laser beamhas its plane of polarization being circularly polarized in theperipheral region B. The laser beam is focused onto the multilayerinformation recording medium 1 with its parts passing through theregions A and B being in different states of polarization. The laserbeam from the central region A is reflected from the multilayerinformation recording medium 1 as the p-polarized light, and is againincident on the turning part 31 to pass therethrough as the p-polarizedlight. On the other hand, the circularly polarized laser beam from theperipheral region B is focused onto the multilayer information recordingmedium 1 to be reflected therefrom circularly polarized in the reversedirection. The reflected light beam is again incident on the turningpart 31 to be converted into an s-polarized light having a polarizationdirection perpendicular to that of the light beam emitted from thesemiconductor laser 2 toward the multilayer information recording medium1. The laser beams from the central region A and the peripheral region Bare incident on the polarization beam splitter 30. The polarization beamsplitter 30 transmits the p-polarized light and reflects the s-polarizedlight. Therefore, the laser beam from the central region A of theturning part 31 passes through the polarization beam splitter 30, whilethe laser beam from the peripheral region B of the turning part 31 isreflected from the polarization beam splitter 30.

[0111] That is, the laser beam from the peripheral region B of theturning part 31 is incident on the detection lens 9, while the laserbeam from the central region A of the turning part 31 is not. The laserbeam from the peripheral region B of the turning part 31 is gathered bythe detection lens 9 to be received by the light-receiving element 10 sothat a focus position is output therefrom.

[0112] Next, the voltage applied to the central region A and the voltageapplied to the peripheral region B are switched so that the voltagelower than the threshold voltage, that is, the voltage of zero volts, isapplied between the upper and lower electrodes of the central region Aand the voltage higher than the threshold voltage is applied to theperipheral region B. In this case, contrary to the previous case wherethe voltage higher than the threshold voltage is applied to the centralregion A while the voltage of zero is applied to the peripheral regionB, the laser beam from the central region A of the turning part 31 isincident on the light-receiving element 10. A focus position is outputfrom the light-receiving element 10 based on the incident light beam.

[0113] As in the first embodiment, the spherical aberration controlcircuit 13 drives and controls the spherical aberration correction part21 based on an amount of spherical aberration obtained from the outputfocus position signals thus generated, thereby eliminating the sphericalaberration. In the second embodiment, after the spherical aberration isdetected, the voltage lower than the threshold voltage is applied toeach of the,central region A and the peripheral region B of the turningpart 31 so that the turning part 31 functions as a ¼ wave plate. Asystem employing a combination of the ¼ wave plate and the polarizationbeam splitter 30 is highly advantageous in an information recording andreproduction apparatus having a recording system requiring highefficiency of use of light.

[0114]FIG. 12 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a third embodimentof the present invention. In FIG. 12, the same elements as those of FIG.5 are referred to by the same numerals, and a description thereof willbe omitted.

[0115] The optical pickup unit of the third embodiment is different fromthat of the first embodiment in that the turning part 7 is provided inthe optical path from the semiconductor laser 2 to the multilayerinformation recording medium 1. The turning part 7 of the thirdembodiment is substantially equal to that of the first embodiment. Theturning part 7 of the third embodiment is identical to that of the firstembodiment in being formed of the liquid crystal element composed of thecentral region A and the peripheral region B as shown in FIG. 6 so thatvoltage is applied to a selected one of the central region A and theperipheral region B, thereby changing the direction of polarization ofthe light beam passing through the central region A or the peripheralregion B.

[0116] In the case of recording information on or reproducinginformation from the multilayer information recording medium 1, first, avoltage higher than a threshold voltage is applied between the upper andlower electrodes of the central region A, while a voltage of zero voltsis applied to the peripheral region B. Thereby, of the light beampassing through the turning part 7, a light beam passing through thecentral region A has its plane of polarization remaining unchanged,while a light beam passing through the peripheral region B has its planeof polarization turned 90°. Under this setting condition, thep-polarized light beam is emitted from the semiconductor laser 2.

[0117] Since the voltage higher than the threshold voltage is applied tothe central region A, the incident laser beam passes therethrough as thep-polarized light with its plane of polarization remaining unchanged. Onthe other hand, since the voltage of zero volts is applied to theperipheral region B, the laser beam passing therethrough has its planeof polarization turned by 90° to be emitted therefrom as an s-polarizedlight. Then, the laser beam passing through the turning part 7 isreflected from the multilayer information recording medium 1 anddeflected by the beam splitter 4 to be incident on the polarizing plate8. The polarizing plate 8 transmits the p-polarized reflected light.Therefore, the laser beam from the central region A of the turning part7 passes through the polarizing plate 8, while the s-polarized laserbeam from the peripheral region B of the turning part 7 is preventedfrom passing through the polarizing plate 8. That is, the laser beamfrom the central region A of the turning part 7 passes through thepolarizing plate 8 as it is to be incident on the detection lens 9,while the laser beam from the peripheral region B of the turning part 7is prevented from being incident on the detection lens 9. The laser beamfrom the central region A of the turning part 7 is gathered by thedetection lens 9 and received by the light-receiving element 10 so thata focus position is output therefrom.

[0118] Next, the voltage applied to the central region A and the voltageapplied to the peripheral region B are switched so that the voltage ofzero volts is applied between the upper and lower electrodes of thecentral region A and the voltage higher than the threshold voltage isapplied to the peripheral region B. In this case, contrary to theprevious case where the voltage higher than the threshold voltage isapplied to the central region A and the voltage of zero volts is appliedto the peripheral region B, the laser beam from the peripheral region BQf the turning part 7 is incident on the detection lens 9. A focusposition is output from the light-receiving element 10 based on theincident light beam.

[0119] As in the first embodiment, the spherical aberration controlcircuit 13 drives and controls the spherical aberration correction part12 based on an amount of spherical aberration obtained from the outputfocus position signals thus generated, thereby eliminating the sphericalaberration.

[0120]FIG. 13 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a fourthembodiment of the present invention. The optical pickup unit of thefourth embodiment includes a turning and spherical aberration correctionpart 40 and a focus driver 41 that performs focus control on an actuator(not shown in the drawing) moving the objective lens 6 in focus andtracking directions.

[0121] The turning and spherical aberration correction part 40 has theglass plates 7 b and 7 c with the transparent electrodes and the TNliquid crystal 7 a sandwiched, in the direction of the optical axis,between the glass plates 7 b and 7 c as shown in FIG. 7. Further, theglass plate 7 b or 7 c has the concentric transparent electrode patternsformed thereon. Such switching between the central region A and thespherical region B as illustrated in FIG. 6 and such selection of anamount of reduction of the beam spot diameter as illustrated in FIGS. 8and 9A through 9G can be performed by selecting electrode patterns towhich voltage are to be applied.

[0122] The multilayer information recording medium 1 is rotated by aninformation recording medium holding part (not shown in the drawing) andan information recording medium rotation control device (not shown inthe drawing). The objective lens actuator is provided in the opticalpickup unit to oppose the multilayer information recording medium 1. Theobjective lens actuator drives the objective lens 6 so that the laserbeam is converged on a recording layer selected based on informationread from the multilayer information recording medium 1 or aninstruction by a user. The turning and spherical aberration correctionpart 40 is provided in part of the optical system including theobjective lens actuator. The beam spot of the laser beam on themultilayer information recording medium 1 is optimized by driving andcontrolling the turning and spherical aberration correction part 40.

[0123] The objective lens actuator is driven and controlled by the focusdriver 41. A focus jump signal is supplied from the control part of theobjective lens actuator to the focus driver 41 based on an operationinstruction in order to reproduce information from a desired recordinglayer.

[0124] In the process of performing focus search according to thisembodiment, the spherical aberration control circuit 13 determines anamount of aberration correction by using such a method of generating asignal (spherical aberration signal) based on the difference between thefocus positions of the central region A and the peripheral region B ofthe liquid crystal element as described in the first through thirdembodiments. The spherical aberration control circuit 13 supplies theamount of aberration correction corresponding to the recording layerthat the information is to be recorded on or reproduced from to theturning and spherical aberration correction part 40 in the form ofapplication of given voltages.

[0125] According to the above-described configuration, in the case ofreproducing information from a desired recording layer of the multilayerinformation recording medium 1, an amount of aberration correctioncorresponding to the desired recording layer is detected and sphericalaberration correction is performed by applying given voltages based onthe detection signal. Further, while the turning part 7 and thespherical aberration correction part 12 are separately provided in theoptical pickup unit in each of the first and third embodiments, thefunctions of the turning part 7 and the spherical aberration correctionpart 12 are realized by the single unit of the turning and sphericalaberration correction part 40 having the liquid crystal having the twofunctions in the fourth embodiment. By thus integrating the turning part7 and the spherical aberration correction part 12 into the single unit,the cost of components can be reduced. The number of assembly steps isalso decreased so that further cost reduction can be realized.Furthermore, the optical pickup unit can be downsized.

[0126]FIG. 14 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a fifth embodimentof the present invention. The optical pickup unit of the fifthembodiment includes a turning and spherical aberration correction part50. In FIG. 14, the same elements as those of FIGS. 5 and 10 arereferred to by the same numerals, and a description thereof will beomitted.

[0127] The turning and spherical aberration correction part 50 iscomposed of a liquid crystal element 50 a and the polarizing plate 8 forselectively transmitting or blocking off a reflected light from arecording layer of the multilayer information recording medium 1 forwhich recording layer the optical characteristics of the objective lens6 are not optimized. The turning and spherical aberration correctionpart 50 is provided in the optical path from the beam splitter 4 and thelight-receiving element 10 as a replacement for the turning part 7 andthe spherical aberration correction part 12 of the first embodiment.

[0128] Transparent electrode patterns are formed concentrically on theliquid crystal 50 a of the turning and spherical aberration correctionpart 50. Like the turning and spherical aberration correction part 40,the turning and spherical aberration correction part 50 is configured sothat such switching between the central region A and the sphericalregion B as illustrated in FIG. 6 and such selection of an amount ofreduction of the beam spot diameter as illustrated in FIGS. 8 and 9Athrough 9G can be performed by selecting electrode patterns to whichvoltages are to be applied.

[0129] First, an amount of spherical aberration for a recording layer ofthe multilayer information recording medium 1 for which layer theoptical characteristics of the objective lens 6 are not optimized isdetected by switching the voltages applied to the turning and sphericalaberration correction part 50 in accordance with the process asdescribed in the first embodiment. Then, given voltages are applied tothe concentric electrode patterns based on the amount of sphericalaberration so that the planes of polarization of light beams passingthrough the respective concentric electrode patterns are different fromeach other. For instance, while a constant voltage is applied to thecentral region of the turning and spherical aberration correction part50, a voltage applied to the concentric spherical region thereof ischanged so as to change the optical phase of the spherical region.

[0130] Then, only a laser beam of a given phase is allowed to passthrough the polarizing plate 8 of the turning and spherical aberrationcorrection part 50 to be received by the light-receiving element 10.

[0131] According to the above-described configuration, of a reflectedlight from a recording layer for which the optical characteristics ofthe objective lens 6 are not optimized, the peripheral part, which issignificantly affected by spherical aberration, is prevented from beingreceived by the light-receiving element 10. This prevents deteriorationof the quality of reproduced signals.

[0132] The present invention is not limited to the specificallydisclosed embodiments of the optical pickup unit of the presentinvention. For instance, spherical aberration is detected by separatelydetecting the focus positions of the central region A and the peripheralregion B according to the spherical aberration method shown in theabove-described embodiments. However, spherical aberration may bedetected by separately detecting the focus position of the sum of thecentral region A and the peripheral region B and the focus position ofthe peripheral region B. Alternatively, the focus positions of aplurality of regions may be detected instead of those of the centralregion A and the peripheral region B. In such a case, a variety ofcombinations of the focus positions are available so that correction canbe made more precisely.

[0133]FIG. 15 is a perspective view of an information recording andreproduction apparatus according to a sixth embodiment of the presentinvention. The information recording and reproduction apparatus of FIG.15 includes an optical disk 60 that is a multilayer informationrecording medium, a cartridge 61 accommodating the optical disk 60, ashutter 62 provided to the cartridge 61 to be openable (and closable) sothat the recording surface may be exposed externally, a dust-proof case63 serving as the exterior of the information recording and reproductionapparatus, an opening part 64 formed on the dust-proof case 63 so thatthe cartridge 61 is inserted thereinto or extracted therefrom throughthe opening part 64, a spindle motor 65 rotating the optical disk 60, acarriage 66 provided with an optical pickup unit, and a carriage movingmechanism 67 moving the carriage 66 in the radial direction of theoptical disk 60.

[0134] In FIG. 15, a variety of signal processing circuits and input andoutput terminals, which are practically necessary components in theinformation recording and reproduction apparatus, are not shown.

[0135] The optical pickup unit supported on the carriage 66 is any ofthe optical pickup units of FIGS. 5 and 10 through 14. Information isrecorded on or reproduced from the optical disk 60 by this opticalpickup unit.

[0136] The information recording and reproduction apparatus is thusconfigured so as to be capable of recording information on andreproducing information from a multilayer information recording medium.

[0137]FIG. 16 is a schematic diagram showing a configuration of anoptical system of an optical pickup unit according to a seventhembodiment of the present invention. The optical pickup unit of FIG. 16includes a semiconductor laser 301 that is a light source, a collimatorlens 302, a first electro-optical element 303 having a transmittancechange region and an aberration correction region, a polarization beamsplitter 304, a deflection mirror 305, a ¼ wave plate 306, an objectivelens 307, a second electro-optical element 309 having a transmittancechange region and an aberration detection signal generation region, adetection lens 310, a light-receiving element 311 that is an elementdetecting a light beam, and a control circuit 312 controlling the firstand second electro-optical elements 303 and 309.

[0138] As shown in FIG. 16, a light beam emitted from the semiconductorlaser 301 is converted into substantially parallel rays by thecollimator lens 302 to be incident on the first electro-optical element303 provided behind (subsequent to) the semiconductor laser 301 in theoptical path between the semiconductor laser 301 and an informationrecording medium 308. As will be described later, the firstelectro-optical element 303 is composed of a first region that is thetransmittance change region switching a transmittance for recordinginformation on the information recording medium 308 and a transmittancefor reproducing information therefrom and a second region that is theaberration correction region performing aberration correction byproviding the light beam passing therethrough with a phase reverse tothe wavefront aberration of the light beam. The light beam passingthrough the first electro-optical element 303 passes through thepolarization beam splitter 304 to have its optical path deflected 90° bythe deflection mirror 305. Thereafter, the light beam is collected bythe objective lens 307. The ¼ wave plate 306 is provided between thedeflection mirror 305 and the objective lens 307. The ¼ wave plate 306coverts the light beam from a linearly polarized light to a circularlypolarized light so that the light beam is focused onto the informationrecording medium 308 circularly polarized.

[0139] The reflected light beam from the information recording medium308 travels through the optical path of the emitted light beam in theopposite direction to again reach the polarization beam splitter 304 viathe objective lens 307, the ¼ wave plate 306, and the deflection mirror305. The light beam reflected from the information recording medium 308to be incident on the ¼ wave plate is circularly polarized in a reversedirection compared with the light beam emitted from the ¼ wave plate 306toward the information recording medium 308. The reflected light beampasses through the ¼ -wave plate 306 to be converted into a linearlypolarized light whose plane of polarization is perpendicular to that ofthe linearly polarized light beam emitted from the semiconductor laser301. The linearly polarized reflected light beam is reflected from thepolarization beam splitter 304, which transmits the light beam emittedfrom the semiconductor laser 301. The reflected light beam reflectedfrom the polarization beam splitter 304 is incident on the secondelectro-optical element 309 provided in front of (preceding) thelight-receiving element 311 in the optical path between the informationrecording medium 308 and the light-receiving element 311. As will bedescribed later, the second electro-optical element 309 is composed of athird region that is the transmittance change region changing itstransmittance at the time of recording and reproduction depending on thetype of the information recording medium 8 and a fourth region that isthe aberration detection signal generation region switching ON or OFFtransmittance for the light beam passing therethrough in order togenerate an aberration detection signal.

[0140] The light beam passing through the second electro-optical element309 passes through the detection lens 310 to reach the light-receivingelement 311. The light-receiving element is suitably divided inaccordance with a servo signal generation method. The reflected lightfrom the information recording medium 308 is detected by thelight-receiving element 311 to be output to other succeeding controlcircuits (not shown in the drawing) as a tracking signal, a focussignal, and a reproduction signal.

[0141] In the conventional optical pickup unit, at a time of recordinginformation on (writing information to) an information recording medium,the output of a semiconductor laser, which is set to a high level, ishardly affected by a returning light. On the other hand, at a time ofreproducing (reading) information from the information recording medium,the output of the semiconductor laser is set to a low level. Therefore,the output of the semiconductor laser is subject to the effect of thereturning light in the conventional optical pickup unit.

[0142] According to the configuration of the present invention, comparedwith the conventional configuration, the output of the semiconductorlaser 301 can be increased at the time of reproduction so that thesemiconductor laser 301 can emit light with power a few milliwattsgreater than the threshold current level without changing the amount oflight reaching the information recording medium 308. By performing suchcontrol, the output of the semiconductor laser 301 can be set to a levelhigher than the conventional level at the time of reproduction, so thatthe semiconductor laser 301 is less affected by the returning light.Further, by setting the transmittance of the first electro-opticalelement 303 to a low rate (value), a rate of returning light to thesemiconductor laser 301 is reduced, so that the light emission of thesemiconductor laser 1 is stabilized and noise in the output of thesemiconductor laser 301 is reduced.

[0143]FIG. 17 is a sectional view of the first electro-optical element303. The first electro-optical element 303 includes a pair of glasssubstrates 315 a and 315 b. Transparent electrodes 316 a and 316 b areformed of ITO (In₂O₃.SnO₂) on the glass substrates 315 a and 315 b,respectively. Polyimide alignment films 317 a and 317 b, which have beensubjected to alignment processing by rubbing, are formed on the ITOtransparent electrodes 316 a and 316 b, respectively.

[0144] A gap material (not shown in the drawing) is provided between thepaired glass substrates 315 a and 315 b so that the paired glasssubstrates 315 a and 315 b oppose each other with a given distancetherebetween. A space between the paired glass substrates 315 a and 315b is sealed with a seal material (not shown in the drawing). A givenliquid crystal is sealed into the space so as to form a liquid crystallayer 318.

[0145] The first electro-optical element 303 is formed as a liquidcrystal cell. In this liquid crystal cell, the alignment of liquidcrystal molecules in the liquid crystal layer 318 is continuouslytwisted 90° between the lower and upper alignment layers 317 a and 317 bwhen alignment processing is performed so that the lower and upperalignment films 317 a and 317 b are 90° different in directions in whichthe longitudinal axes of liquid crystal molecules are aligned, that is,when the lower and upper alignment films 317 a and 317 b are rubbed indirections 90° different from each other. Further, as shown in FIG. 17,a polarizing film 319, for instance, is deposited on the liquid crystalcell on the side opposite to the semiconductor laser 301 opposing theliquid crystal cell so as to cover a region corresponding to thetransmittance change region. The polarizing film 319 is provided so thatits polarization axis coincides with the direction of alignment ofliquid crystal molecules on the surface of the adjacent glass substrate315 b.

[0146] According to this arrangement, a relationship between a voltage Vapplied to the liquid crystal cell and its transmittance T is as shownin FIG. 18. That is, when a low voltage is applied to the liquid crystalcell, the plane of polarization of the light beam emitted from thesemiconductor laser 301 to be incident on the liquid crystal cell isturned 90° in accordance with the twist of the liquid crystal moleculesto be absorbed into the polarizing film 319.

[0147] That is, in the case of applying a low voltage to the liquidcrystal cell, transmittance for the light beam emitted from thesemiconductor laser 301 is controlled to a low rate (value). On theother hand, when a high voltage is applied to the liquid crystal cell,the twist of the alignment of the liquid crystal molecules disappears.As a result, the light beam emitted from the semiconductor laser 301 andincident on the liquid crystal cell travels straight without its planeof polarization being turned to pass through the polarizing film 319.That is, in the case of applying a high voltage to the liquid crystalcell, the light beam emitted from the semiconductor laser 301 iscontrolled to a high rate (value).

[0148] In the case of employing this liquid crystal cell, which canperform such transmittance control, as the first electro-optical element303 in the optical pickup unit, the light-emission power of thesemiconductor laser 301 is set to 35 mW and the transmittance of thefirst electro-optical element 303 is set to 85% at the time ofrecording, for instance, so that the optical system may have a usabilityof light of 40% with respect to the light beam traveling up to theinformation recording medium 308. Therefore, the recording power of thelight beam can be set to approximately 12 mW on the informationrecording medium 308.

[0149] On the other hand, at the time of reproduction, thelight-emission power of the semiconductor laser 301 can be set to eightmilliwatts with the transmittance of the first electro-optical element303 being set to a low rate, for instance, 30%. In this case, thereproduction power of the light beam passing through the same opticalsystem as at the time of recording can be set to approximately onemilliwatt on the information recording medium 308.

[0150] As previously described, the semiconductor laser 301 isconsiderably affected by the returning light while its light-emissionpower is in the range up to approximately five milliwatts. According tothe present invention, however, the semiconductor laser 301 is allowedto emit the light beam with a sufficient light-emission power of, forinstance, eight milliwatts at the time of reproduction. Therefore, noiseeffects can be avoided without employing a high-frequency superimposedcircuit.

[0151] As shown in FIG. 19, the electro-optical element 303 has theaberration correction region for correcting wavefront aberration (thesecond region) provided outside the transmittance change region (thefirst region). In the second region, at least one of the transparentelectrodes 16 a and 16 b of the electro-optical element 303 is dividedaccording to a given pattern. A voltage applied to each dividedelectrode part is variably controlled based on the later-describedaberration detection signal so that the refractive index of each dividedpart is changed to provide a phase difference to a light beam (part ofthe light beam) passing through the divided part. Thereby, the wavefrontaberration including coma and spherical aberration of the objective lens307 can be corrected. That is, by controlling the voltages applied tothe divided second region, the refractive index n of the liquid crystalof each divided part of the second region can be varied freely from n1to n2.

[0152] The fact that the refractive index n is variable means that thelight beam passing through each divided part of the second region can beprovided with an optical path difference Δn·d (Δn is a variation of therefractive index and d is a liquid crystal cell thickness), that is, aphase difference Δn·d(2π/λ) (λ is the wavelength of the light beam).Thus, by controlling the applied voltages in accordance with wavefrontaberration occurring in the objective lens 307 so as to change therefractive index n of each divided part of the second region, wavefrontaberration caused by the objective lens 307 can be corrected.

[0153]FIGS. 20A through 20C are diagrams showing typical types ofwavefront aberration. FIGS. 20A through 20C show spherical aberration,coma, and astigmatism, respectively. As is apparent from FIGS. 20Athrough 20C, the peripheral part of a light beam passing through anobjective lens is significantly affected by wavefront aberration, whilea returning light affects the central region of the light beam, wherethe power of the light beam is high. Thus, stable control of thesemiconductor laser 301 and correction of the wavefront aberration ofthe light beam incident on the objective lens 307 can be performedcompatibly by the single electro-optical element 303 by dividing theregion thereof.

[0154] Further, in the optical pickup unit of FIG. 16, the intensity ofthe light beam (signal light) incident on the light-receiving element311 is extremely high at the time of recording information on theinformation recording medium 308 since the output power of thesemiconductor laser 301 is set to a high level. On the other hand, atthe time of reproducing information from the information recordingmedium 308, the output power of the semiconductor laser 301 is set to alow level so that the intensity of the light beam is low. Recently,there have been a plurality of types of recording media that areemployable as the information recording medium 308, such as a ROMmedium, a write-once medium, and a phase-change medium. Reflectivity isdifferent in each type of recording medium. Thus, an optimum signallevel differs depending on an operation (that is, whether recording orreproduction is performed) or a medium type.

[0155] If the amplitude level of a signal is too low, a problem iscaused in information reproduction. Therefore, the gain of acurrent-voltage conversion amplifier succeeding the light-receivingelement 311 is switchable so that the output amplitude of thecurrent-voltage conversion amplifier falls within a proper range.However, there has been a necessity of considering the type of theinformation recording medium 308 or a plurality of conditions ofrecording and reproduction. For instance, in the case of making theconventional gain switching circuit as shown in FIG. 4 suitable for aplurality of condition patterns, the number of resistors attached to thecircuit increases so that a signal response speed is decreased.

[0156] According to the present invention, at the time of reproduction,an amount of light reflected from the information recording medium 308remains unchanged from that in the conventional configuration because ofthe second electro-optical element 309, while at the time of recording,the transmittance of the second electro-optical element 309 for thelight beam reflected from the information recording medium 308 is set toa low rate. Thereby, the gain of the current-voltage conversionamplifier remains the same at the time of recording and at the time ofreproduction. Further, the dynamic range of the light-receiving element311 is restricted so that the output of the light-receiving element 311is saturated if the input level is too high. According to the presentinvention, an amount of light directed onto the light-receiving element311 itself can be controlled by the second electro-optical element 309so that restriction resulting from the dynamic range can be relaxed.

[0157] The second electro-optical element 309 has the same configurationas the first electro-optical element 303 shown in FIG. 17. Further, thesecond electro-optical element 309 has the aberration detection signalgeneration region (the fourth region) provided outside the transmittancechange region (the third region). The fourth region includes a shutterfunction for generation of the aberration detection signal. The fourthregion, which is a transmittance change part of the same configurationas the third region, has a transparent electrode divided according tothe aberration detection signal to be generated.

[0158] As shown in FIG. 21, which is a diagram showing occurrence ofspherical aberration, spherical aberration is a phenomenon that thefocus position of part of a light beam around the axis of its beam spotis different from that of a peripheral part of the light beam. Byseparating the two parts of the light beam and detecting the focusposition of each of the two parts separately, an amount of sphericalaberration can be obtained from a difference between the focuspositions. On the other hand, an amount of light to the light-receivingelement 311 is required to be controlled in the high-power centralregion of the light beam. Therefore, stable signal detection andgeneration of the aberration detection signal for aberration correctioncan be realized by the single element irrespective of the operationperformed (recording or reproduction) and the medium type by dividingthe region of the element.

[0159] A description will now be given of specific examples ofaberration correction and detection by an electro-optical elementemployed as the first and second electro-optical elements 303 and 309.Needless to say, transmittance for the light beam emitted from the lightsource (semiconductor laser 301) is changed in the optical path towardthe information recording medium 308 (a lighting optical path) and theamount of light directed onto the light-receiving element 311 iscontrolled in the optical path from the information recording medium 308(a detection optical path).

[0160] If there is a variation in the thickness of the informationrecording medium 308, spherical aberration occurs when the light beampasses through the substrate of the information recording medium 308. Ina first example is shown a configuration of the electro-optical elementin the case of correcting this spherical aberration.

[0161] The spherical aberration is detected by the light-receivingelement 311 and the electro-optical element is actuated to cancel thisspherical aberration, so that the spherical aberration is corrected.FIG. 22A is a diagram showing wavefront aberration when sphericalaberration occurs. The wave surface of the light beam includes delays 23a and 23 b with respect to a reference wave surface 22. The delays 23 aand 23 b occur symmetrically with respect to an optical axis 21. Whenthe reference wave surface 22 is focused, a position at which thedelayed wave surface is focused is a defocus with respect to the focuspoint of the reference wave surface 22.

[0162] Therefore, the occurrence of the spherical aberration can beunderstood by detecting a focus condition by obtaining a differencebetween the delayed wave surface and the reference wave surface 22. Forinstance, if the electro-optical element has a pattern shown in FIG.22B, the aberration detection signal of the spherical aberration can begenerated based on the signal of the light-receiving element 311obtained by transmitting the light beam selectively through an A region24 and a B region 25.

[0163] Next, a description will be given of a configuration forcorrecting spherical aberration by the electro-optical element based onthe aberration detection signal. FIG. 23A is a diagram showing a sectionof the spherical aberration of FIG. 20A. As shown in FIG. 20A, thespherical aberration becomes greater in proportion to the distance fromthe optical axis. Therefore, the spherical aberration can be canceled bythe electro-optical element (liquid crystal cell) providing the lightbeam passing therethrough with a phase difference indicated by a brokenline in FIG. 23A, which phase difference is reverse to that indicated bythe solid line in FIG. 23A. FIG. 23B is a diagram showing the sphericalaberration after correction, which is the sum of the solid and brokenlines of FIG. 23A. FIG. 23B shows that the spherical aberration isconsiderably reduced compared with its original amount. FIG. 24A is adiagram showing a pattern for correcting the spherical aberration in theelectro-optical element.

[0164] If the information recording medium 308 is inclined to have atilt, coma is generated when the light beam passes through the substrateof the information recording medium 308. In a second example is shown aconfiguration of the electro-optical element in the-case of correctingthis coma.

[0165] The coma is detected by the light-receiving element 311 so thatthe electro-optical element is driven to cancel the coma. Thereby, thecoma is corrected. FIG. 25A is a diagram showing wavefront aberrationwhen the coma occurs. The wave surface of the light beam includes anadvance 26 a and a delay 26 b with respect to its reference wave surface22. When the reference wave surface 22 is focused, a position at whicheach of the advanced and delayed wave surfaces is focused is a defocuswith respect to the focus point of the reference wave surface 22.

[0166] Therefore, the occurrence of the coma can be understood bydetecting a focus condition by obtaining a difference between each ofthe advanced and delayed wave surfaces and the reference wave surface22. For instance, if the electro-optical element has a pattern shown inFIG. 25B, the aberration detection signal of the coma can be generatedbased on the signal of the light-receiving element 311 obtained bytransmitting the light beam selectively through an A₁ region 27 and a B₁region 29, and an A₂ region 28 and a B₂ region 30.

[0167]FIG. 26A is a diagram showing a section of the coma caused by thetilt. In this case as well, the coma due to the tilt can be canceled asshown in FIG. 26B by the electro-optical element (liquid crystal cell)providing the light beam passing therethrough with a phase differenceindicated by a broken line in FIG. 26A. FIG. 24B is a diagram showing apattern for correcting the coma in the electro-optical element.

[0168] When the light beam passes through the substrate of theinformation recording medium 308, astigmatism is generated due to doublerefraction caused by the information recording medium 308. In a thirdexample is shown a configuration of the electro-optical element forcorrecting this astigmatism.

[0169] The astigmatism is detected by the light-receiving element 311 sothat the electro-optical element is driven to cancel the astigmatism.Thereby, the astigmatism is corrected. Detection of the astigmatism canbe performed based on the same idea as detection of the above-describedspherical aberration and coma. FIG. 24C is a diagram showing a patternfor correcting the astigmatism in the electro-optical element.

[0170] Further, a plurality of types of aberration can be eliminated atthe same time by combining the patterns of FIG. 24A through 24C.Furthermore, the above-described electro-optical element may beincorporated into the first and second electro-optical elements 303 and309 of FIG. 16 as the second and fourth regions thereof.

[0171] The optical pickup unit of the seventh embodiment of the presentinvention can be provided in the information recording and reproductionapparatus of FIG. 15. That is, an information recording and reproductionapparatus according to the seventh embodiment of the present inventioncan be realized by providing the optical pickup unit of the seventhembodiment in the information recording and reproduction apparatus ofFIG. 15.

[0172] The present invention is not limited to the specificallydisclosed embodiments, but variations and modifications may be madewithout departing from the scope of the present invention.

[0173] The present application is based on Japanese priority patentapplications No. 2001-178502filed on Jun. 13, 2001 and No. 2001-371079filed on Dec. 5, 2001, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. An optical pickup unit comprising: a light sourceemitting a light beam; an objective lens focusing the light beam onto aninformation recording medium; a light detection part receiving the lightbeam reflected from the information recording medium; and a lightblocking part selectively blocking a part of the light beam with respectto a radial direction, the light blocking part provided in an opticalpath of the light beam to be centered on an optical axis.
 2. The opticalpickup unit as claimed in claim 1, wherein said light blocking partcomprises: a turning part selectively turning a plane of polarization ofthe part of the light beam; and a transmission part selectively blockingthe part of the light beam based on a direction of polarization of thepart of the light beam, the light beam being incident on saidtransmission part after being emitted from said turning part.
 3. Theoptical pickup unit as claimed in claim 2, wherein said turning part isa liquid crystal element.
 4. The optical pickup unit as claimed in claim2, wherein said transmission part is a polarizing plate.
 5. The opticalpickup unit as claimed in claim 2, wherein said transmission part is apolarizing film formed on said turning part.
 6. The optical pickup unitas claimed in claim 2, wherein said transmission part is a polarizationbeam splitter.
 7. The optical pickup unit as claimed in claim 2, whereinsaid turning part comprises first and second regions formed thereon sothat the first and second regions provide different planes ofpolarization to the light beam passing through said turning part.
 8. Theoptical pickup unit as claimed in claim 7, wherein the first and secondregions of the said turning part are formed of transparent electrodepatterns centered on the optical axis, so that light beams are emittedfrom the first and second regions of said turning part concentricallywith each other.
 9. The optical pickup unit as claimed in claim 2,wherein said turning part selectively turns the plane of polarization ofthe part of the light beam based on a voltage applied to said turningpart.
 10. The optical pickup unit as claimed in claim 2, furthercomprising a control part generating an aberration signal by comparing afirst signal generated based on a first part of the light beam passingthrough a first region of said light blocking part and a second signalgenerated based on a second part of the light beam passing through asecond region of said light blocking part, the first region of saidlight blocking part being provided internal to the second regionthereof.
 11. The optical pickup unit as claimed in claim 10, furthercomprising a spherical aberration correction part correcting sphericalaberration based on the aberration signal generated by said controlpart.
 12. The optical pickup unit as claimed in claim 11, wherein saidturning part and said spherical aberration correction part comprise asingle element.
 13. The optical pickup unit as claimed in claim 1,wherein said light blocking part comprises a liquid crystal element. 14.The optical pickup unit as claimed in claim 1, further comprising acontrol part generating an aberration signal by comparing a first signalgenerated based on a first part of the light beam passing through afirst region of said light blocking part and a second signal generatedbased on a second part of the light beam passing through a second regionof said light blocking part, the first region of said light blockingpart being provided internal to the second region thereof.
 15. Theoptical pickup unit as claimed in claim 14, wherein focus positions ofthe first and second regions of said light blocking part are comparedbased on the first and second signals.
 16. The optical pickup unit asclaimed in claim 14, further comprising a spherical aberrationcorrection part correcting-spherical aberration based on the aberrationsignal generated by said control part.
 17. The optical pickup unit asclaimed in claim 16, wherein said spherical aberration correction partis a liquid crystal element.
 18. The optical pickup unit as claimed inclaim 16, wherein said light blocking part and said spherical aberrationcorrection part comprise a single element.
 19. The optical pickup unitas claimed in claim 16, wherein said spherical aberration correctionpart comprises a plurality of concentric regions formed of transparentelectrode patterns around the optical axis, the concentric regionsproviding different phases to the light beam passing through saidspherical aberration correction part so as to correct the sphericalaberration of the objective lens.
 20. The optical pickup unit as claimedin claim 16, wherein said spherical aberration correction part comprisesa mechanism moving said objective lens so as to correct the sphericalaberration.
 21. The optical pickup unit as claimed in claim 20, whereinsaid mechanism comprises a piezoelectric element.
 22. The optical pickupunit as claimed in claim 20, wherein said mechanism comprises anelectromagnetically driven actuator.
 23. The optical pickup unit asclaimed in claim 1, further comprising a spherical aberration correctionpart correcting spherical aberration.
 24. The optical pickup unit asclaimed in claim 23, wherein said spherical aberration correction partis a liquid crystal element.
 25. The optical pickup unit as claimed inclaim 23, wherein said light blocking part and said spherical aberrationcorrection part comprise a single element.
 26. The optical pickup unitas claimed in claim 1, wherein said light blocking part selectivelyblocks a first part of the light beam and transmits a second partthereof so that the second part of the light beam is received by saidlight detection part.
 27. The optical pickup unit as claimed in claim26, wherein said light blocking part further comprises: a turning partturning a plane of polarization of the first part of the light beam andtransmitting the second part of the light beam without turning a planeof polarization thereof; and a transmission part blocking the first partof the light beam and transmitting the second part of the light beambased on respective directions of polarization of the first and secondparts of the light beam, the light beam being incident on saidtransmission part after being emitted from said turning part.
 28. Theoptical pickup unit as claimed in claim 1, wherein said light blockingpart selectively blocks the part of the light beam based on a voltageapplied to said light blocking part.
 29. The optical pickup unit asclaimed in claim 1, wherein the information recording medium is amultilayer information recording medium including a plurality ofrecording layers.
 30. An information recording and reproductionapparatus, comprising: an optical pickup unit, the optical pickup unitcomprising: a light source emitting a light beam; an objective lensfocusing the light beam onto an information recording medium; a lightdetection part receiving the light beam reflected from the informationrecording medium; and a light blocking part selectively blocking a partof the light beam with respect to a radial direction, the light blockingpart provided in an optical path of the light beam to be centered on anoptical axis.
 31. The information recording and reproduction apparatusas claimed in claim 30, wherein said light blocking part comprises: aturning part selectively turning a plane of polarization of the part ofthe light beam; and a transmission part selectively blocking the part ofthe light beam based on a direction of polarization of the part of thelight beam, the light beam being incident on said transmission partafter being emitted from said turning part.
 32. The informationrecording and reproduction apparatus as claimed in claim 31, whereinsaid turning part comprises first and second regions formed thereon sothat the first and second regions provide different planes ofpolarization to the light beam passing through said turning part. 33.The information recording and reproduction apparatus as claimed in claim32, wherein the first and second regions of the said turning part areformed of transparent electrode patterns centered on the optical axis,so that light beams are emitted from the first and second regions ofsaid turning part concentrically with each other.
 34. The informationrecording and reproduction apparatus as claimed in claim 31, whereinsaid turning part selectively turns the plane of polarization of thepart of the light beam based on a voltage applied to said turning part.35. The information recording and reproduction apparatus as claimed inclaim 31, wherein said optical pickup unit further comprises a controlpart generating an aberration signal by comparing a first signalgenerated based on a first part of the light beam passing through afirst region of said light blocking part and a second signal generatedbased on a second part of the light beam passing through a second regionof said light blocking part, the first region of said light blockingpart being provided internal to the second region thereof.
 36. Theinformation recording and reproduction apparatus as claimed in claim 35,wherein said optical pickup unit further comprises a sphericalaberration correction part correcting spherical aberration based on theaberration signal generated-by said control part.
 37. The informationrecording and reproduction apparatus as claimed in claim 36, whereinsaid turning part and said spherical aberration correction part comprisea single element.
 38. The information recording and reproductionapparatus as claimed in claim 30, wherein said light blocking partcomprises a liquid crystal element.
 39. The information recording andreproduction apparatus as claimed in claim 30, wherein said opticalpickup unit further comprises a control part generating an aberrationsignal by comparing a first signal generated based on a first part ofthe light beam passing through a first region of said light blockingpart and a second signal generated based on a second part of the lightbeam passing through a second region of said light blocking part, thefirst region of said light blocking part being provided internal to thesecond region thereof.
 40. The information recording and reproductionapparatus as claimed in claim 39, wherein focus positions of the firstand second regions of said light blocking part are compared based on thefirst and second signals.
 41. The information recording and reproductionapparatus as claimed in claim 391 wherein said optical pickup unitfurther comprises a spherical aberration correction part correctingspherical aberration based on the aberration signal generated by saidcontrol part.
 42. The information recording and reproduction apparatusas claimed in claim 41, wherein said spherical aberration correctionpart is a liquid crystal element.
 43. The information recording andreproduction apparatus as claimed in claim 41, wherein said lightblocking part and said spherical aberration correction part comprise asingle element.
 44. The information recording and reproduction apparatusas claimed in claim 41, wherein said spherical aberration correctionpart comprises a plurality of concentric regions formed of transparentelectrode patterns around the optical axis, the concentric regionsproviding different phases to the light beam passing through saidspherical aberration correction part so as to correct the sphericalaberration of the objective lens.
 45. The information recording andreproduction apparatus as claimed in claim 41, wherein said sphericalaberration correction part comprises a mechanism moving said objectivelens so as to correct the spherical aberration.
 46. The informationrecording and reproduction apparatus as claimed in claim 30, whereinsaid optical pickup unit further comprises a spherical aberrationcorrection part correcting spherical aberration.
 47. The informationrecording and reproduction apparatus as claimed in claim 46, whereinsaid spherical aberration correction part is a liquid crystal element.48. The information recording and reproduction apparatus as claimed inclaim 46, wherein said light blocking part and said spherical aberrationcorrection part comprise a single element.
 49. The information recordingand reproduction apparatus as claimed in claim 30, wherein said lightblocking part selectively blocks the part of the light beam based on avoltage applied to said light blocking part.
 50. The informationrecording and reproduction apparatus as claimed in claim 30, wherein theinformation recording medium is a multilayer information recordingmedium including a plurality of recording layers; focus jump isperformed to a desired one of the recording layers that information isto be recorded on or reproduced from so that an amount of sphericalaberration with respect to the desired one of the recording layers isdetected; and the spherical aberration is corrected based on thedetected amount thereof.
 51. An optical pickup unit performing recordingor reproducing information by making a light beam emitted from a lightsource incident on an information recording medium, the optical pickupunit comprising: an electro-optical element switching values oftransmittance of a given region through which the light beam passesdepending on whether the information is recorded or reproduced.
 52. Theoptical pickup unit as claimed in claim 51, wherein said electro-opticalelement is provided subsequent to the light source in an optical pathbetween the light source and the information recording medium; and thetransmittance of the given region of said electro-optical element iscontrolled to a first value at a time of reproducing the informationfrom the information recording medium and to a second value at a time ofrecording the information on the information recording medium, the firstvalue being smaller than the second value.
 53. The optical pickup unitas claimed in claim 52, wherein said electro-optical element comprisesan outer region provided outside the given region so that the light beampassing through the outer region has a phase difference thereofselectively varied in the outer region.
 54. The optical pickup unit asclaimed in claim 53, wherein the light beam passing through the outerregion has the phase difference thereof selectively variedconcentrically in the outer region.
 55. The optical pickup unit asclaimed in claim 53, wherein the light beam passing through the outerregion has the phase difference thereof varied in the outer region stepby step in a radial or tangential direction.
 56. The optical pickup unitas claimed in claim 53, wherein the light beam passing through the outerregion has the phase difference thereof varied in the outer regionsimultaneously and asymmetrically in radial and tangential directions.57. The optical pickup unit as claimed in claim 53, wherein the outerregion includes a transparent electrode divided into a plurality ofparts; and the phase difference of the light beam passing through theouter region is selectively varied therein by controlling voltagesapplied to the divided parts so as to vary refractive indices thereof.58. The optical pickup unit as claimed in claim 51, wherein saidelectro-optical element is provided preceding a detection elementreceiving the light beam reflected from the information recording mediumin an optical path between the information recording medium and thedetection element; and the transmittance of the given region of saidelectro-optical element is controlled to a first value at a time ofreproducing the information from the information recording medium and toa second value at a time of recording the information on the informationrecording medium, the first value being larger than the second value.59. The optical pickup unit as claimed in claim 58, wherein saidelectro-optical element comprises an outer region provided outside thegiven region, the outer region controlling a transmittance thereof forthe light beam passing through the outer region; and the detectionelement detects a difference between a focus position of the givenregion and a focus position of the outer region.
 60. The optical pickupunit as claimed in claim 59, wherein the given region and the outerregion are formed concentrically.
 61. The optical pickup unit as claimedin claim 59, wherein the outer region is divided into strip-like partsin a radial or a tangential direction.
 62. An information recording andreproduction apparatus recording information on and reproducinginformation from an information recording medium, the informationrecording and reproduction apparatus comprising: an optical pickup unitperforming recording or reproducing information by making a light beamemitted from a light source incident on the information recordingmedium, the optical pickup unit comprising: an electro-optical elementswitching values of transmittance of a given region through which thelight beam passes depending on whether the information is recorded orreproduced.
 63. The information recording and reproduction apparatus asclaimed in claim 62, wherein said electro-optical element is providedsubsequent to the light source in an optical path between the lightsource and the information recording medium; and the transmittance ofthe given region of said electro-optical element is controlled to afirst value at a time of reproducing the information from theinformation recording medium and to a second value at a time ofrecording the information on the information recording medium, the firstvalue being smaller than the second value.
 64. The information recordingand reproduction apparatus as claimed in claim 63, wherein saidelectro-optical element comprises an outer region provided outside thegiven region so that the light beam passing through the outer region hasa phase difference thereof selectively varied in the outer region. 65.The information recording and reproduction apparatus as claimed in claim64, wherein the light beam passing through the outer region has thephase difference thereof selectively varied concentrically in the outerregion.
 66. The information recording and reproduction apparatus asclaimed in claim 64, wherein the light beam passing through the outerregion has the phase difference thereof varied in the outer region stepby step in a radial or a tangential direction.
 67. The informationrecording and reproduction apparatus as claimed in claim 64, wherein thelight beam passing through the outer region has the phase differencethereof varied in the outer region simultaneously and asymmetrically inradial and tangential directions.
 68. The information recording andreproduction apparatus as claimed in claim 64, wherein the outer regionincludes a transparent electrode divided into a plurality of parts; andthe phase difference of the light beam passing through the outer regionis selectively varied therein by controlling voltages applied to thedivided parts so as to vary refractive indices thereof.
 69. Theinformation recording and reproduction apparatus as claimed in claim 62,wherein said electro-optical element is provided preceding a detectionelement receiving the light beam reflected from the informationrecording medium in an optical path between the information recordingmedium and the detection element; and the transmittance of the givenregion of said electro-optical element is controlled to a first value ata time of reproducing the information from the information recordingmedium and to a second value at a time of recording the information onthe information recording medium, the first value being larger than thesecond value.
 70. The information recording and reproduction apparatusas claimed in claim 69, wherein said electro-optical element comprisesan outer region provided outside the given region, the outer regioncontrolling a transmittance thereof for the light beam passing throughthe outer region; and the detection element detects a difference betweena focus position of the given region and a focus position of the outerregion.
 71. The information recording and reproduction apparatus asclaimed in claim 70, wherein the given region and the outer region areformed concentrically.
 72. The information recording and reproductionapparatus as claimed in claim 70, wherein the outer region is dividedinto strip-like parts in a radial or a tangential direction.